The following processes have been known for preparation of scandium oxide.
(a) A process for preparing scandium oxide comprising causing hydrolysis of an acid aqueous solution containing scandium by adding ammonium hydroxide, sodium hydroxide or the like to form a precipitate of scandium hydroxide, collecting the precipitate and roasting it to obtain scandium oxide. PA0 (b) A process for preparing scandium oxide comprising forming a precipitate of scandium carbonate by adding sodium carbonate, ammonium carbonate or the like to an acid aqueous solution containing scandium, collecting the precipitate and roasting it to obtain scandium oxide. PA0 (c) A process for preparing scandium oxide comprising forming a precipitate of scandium oxalate by adding oxalic acid to an acid aqueous solution containing scandium, collecting the precipitate and roasting it to obtain scandium oxide.
In the processes of (a) and (b), the obtained precipitates are colloidal and their filtration is extremely difficult, and the efficient recovery of scandium oxide is impossible. Further, the scandium oxide obtained by roasting the scandium hydroxide or scandium carbonate is in the coagulated state and it becomes a mixture of coarse particles and extremely fine particles when crushed and pulverized, that is, it cannot be obtained with a uniform particle size distribution.
The scandium oxalate obtained by the process of (c) is easy to filtrate. However, the scandium oxide powder obtained by thermal decomposition of the thus obtained oxalate is planar or columnar.
At any rate, hexagonal-bipyramid crystalline scandium oxide of powder has not been obtained.
We attempted to improve the process (c) in order to obtain scandium oxalate precipitate having good filterability by modifying the composition of the mother liquor and appropriate selection of the oxalic acid compound to be used, and we have found that hexagonal-bipyramid crystalline scandium oxide powder can be obtained by forming scandium oxalate precipitate in the presence of chloride ions and ammonium ions.